Thiadiazoloquinoxalines: Tuning physical properties through smart synthesis

Article abstract of DOI:10.1021/ol200250e

The synthesis of conjugated acceptors based on thiadiazoloquinoxaline (TQ) derivatives is described. Apart from reporting on the functionalization of the TQ core, the influence of the substituents was studied by UV-vis absorption and emission spectroscopy

Full text of DOI:10.1021/ol200250e

ORGANIC
LETTERS
2011
Vol. 13, No. 8
1936–1939
Thiadiazoloquinoxalines: Tuning Physical
Properties through Smart Synthesis
Timea Dallos, Manuel Hamburger, and Martin Baumgarten*
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
martin.baumgarten@mpip-mainz.mpg.de
Received January 26, 2011
ABSTRACT
The synthesis of π-conjugated acceptors based on thiadiazoloquinoxaline (TQ) derivatives is described. Apart from reporting on the
functionalization of the TQ core, the influence of the substituents was studied by UV-vis absorption and emission spectroscopy, cyclic
voltammetry measurements, and DFT calculations. By changing the donor as well as the π-spacer, a fine-tuning of the photo- and electrochemical
properties was achieved.
Organic π-conjugated molecules and polymers are versa-
tile materials for the application in light emitting diodes,1
photovoltaic cells,2 field effect transistors,3 and nonlinear
optics4. Distinct synthetic design in these systems allows the
tailoring of the energy gap between the highest occupied
molecular orbital (HOMO) and the lowest unoccupied mo-
lecular orbital (LUMO).5 The HOMO-LUMO gap de-
termines the optical and electrochemical behavior of π-
conjugated systems therefore their applicability in electronic
devices. An effective synthetic strategy to decrease the
HOMO-LUMO gap involves the alternation of electron-
donating (D) and electron-accepting (A) moieties, within the
engineered molecular backbone, thus implicitly increasing
the conjugation length.6 To maximize the effective π-con-
jugation length, steric interactions between adjacent units
needs to be minimized.7 Hence, within D-A-D type mole-
cules, the introduction of π-spacers such as olefinic or
acetylenic π-spacers enables the generation of structures with
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r
10.1021/ol200250e
Published on Web 03/11/2011
2011 American Chemical Society

increased conjugation, while keeping steric constraints at a
minimum.8 The [1,2,5]thiadiazolo[3,4]quinoxaline (TQ) mo-
lecule is an o-quinoid acceptor unit featuring outstanding
electron affinity and has been used in the construction of low
band gap semiconducting polymers.9 So far, TQ derivatives
were reported in conjugation with neighboring thiophene
units at positions 4 and 9.10
Scheme 2. Synthesis of TQ Derivatives 7a, 8a-c, and 9a
Scheme 1. Synthesis of 4,9-Dibromo-6,7 bis(aryl)-[1,2,5]-
thiadiazolo[3,4-g]quinoxaline
Herein, we report the synthesis and characterization of
new TQ oligomers with neighboring ethynyl and vinyl
spacers. The changes in the optical and electrochemical
behavior originating from the difference in the donor
strength of the substituents at positions 6 and 7 on the
TQ unit, as well as in the nature of π-spacers (ethynyl vs.
vinyl) in the co-oligomers (at positions 4 and 9), are
reported (Figure S1, Supporting Information, SI).
that, after the coupling reaction was completed, the palla-
dium catalyst acted as a hydrogen-transfer catalyst, redu-
cing the triple bond. The hydrogen source might be the
diisopropylammonium salt, resulting from the deprotona-
tion of the terminal acetylene. The presumed Pd-catalyzed
hydrogenation might be facilitated by the strong electron-
withdrawing nature of the TQ core.14
First, the influcence of the substituents directly con-
nected to the TQ core, at positions 6 and 7, was evalua-
ted (Table 1). Compound 6a, carrying phenyl moieties,
showed an absorption maximum at 440 nm and an emis-
sion maximum at 546 nm (Figure S2, SI). Replacing the
phenyl substituents by thiophene (6b) enhanced the D-A
interactions, lowering the optical energy gap (Eop). This
was reflected in red shifts of the absoption and emission
maxima in solution. To determine the electrochemical
properties of the compounds, cyclic voltammetry (CV)
experiments were performed and the first reduction peak
potentials (ERed) relative to ferrocene were measured
(Figure S3, SI). The CV measurements revealed that both
compounds presented similar ELUMO (Table 1) suggesting
that, by the introduction of stronger donor units at posi-
tions 6 and 7, the high electron affinity of the molecule was
preserved.
Diketones 2 and 4 were obtained from the commercially
available dodecylbenzene or octylthiophene under Frie-
del-Crafts reaction conditions in 56% and 72% yield,
respectively.11 The condensation of 512,9b with diketone 2
or 4 was conducted in acetic acid (AcOH), under micro-
wave (MW) irradiation (Scheme 1). Compounds 6a and 6b
were isolated as orange (6a, 72% yield) and red (6b, 54%
yield) precipitates.
Performing the well-established one-pot consecutive
Sonogashira-Hagihara13 cross-couplingreaction between
6a,b and thecorrespondingaromaticterminalalkyneled to
the formation of 7-9a (77-82% yield) in only 15 min
(Scheme 2). Trying to increase the yield of 8a by extending
the reaction time of 6a and 2-ethynylthiophene resulted in
the formation of 8b and 8c, respectively. The highest yields
for 8b (16% yield) and 8c (12% yield) were obtained after
12 h, whereas neither 6b nor 8a was left in the reaction
mixture. The hydrogenation of the alkyne 8a indicated
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Li, H.; Tam, T. L.; Lam, Y. M.; Mhaisalkar, S. G.; Grimsdale, A. C.
Second, the photophysical properties of 7a, 8a, and 9a
(Figure 1, Table 1) could be tuned by varying the donor
strength connected via acetylenic spacers at the 4 and 9
positions.
Org. Lett. 2011, 13, 46.
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A. V.; Strelenko, Y. A.; Tandura, S. N.; Shumskii, A. N.; Krayushkin,
M. M. Chem. Heterocycl. Compd. 2003, 39, 1570.
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T. Heterocycles 1992, 33, 337. (b) Tam, T. L.; Li, H.; Wei, F.; Tan, K. J.;
Kloc, C.; Lam, Y. M.; Mhaisalkar, S. G.; Grimsdale, A. C. Org. Lett.
2010, 12, 3340.
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Org. Lett., Vol. 13, No. 8, 2011
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Table 1. Photophysical and Electrochemical Properties of 6a,b, 7a, 8a-c, and 9a
compd
λ
abs/nm (log ε)a
λ
em/nma (Φ/%b)
λ
abs/nm (λem/nm) filmc Eop/eVa ERed/Vd ELUMO/eVe EHOMO/eVf Ecalc/eVg
6a
6b
7a
8a
8b
8c
9a
440 (4.33)
546 (-)
2.30
2.17
1.98
1.86
1.73
1.64
1.78
-0.630
-0.621
-0.622
-0.611
-0.662
-0.707
-0.592
-3.79
-3.80
-3.79
-3.80
-3.75
-3.70
-3.83
-6.09
-5.97
-5.77
-5.66
-5.48
-5.34
-5.62
2.65
2.56
2.02
1.89
1.79
1.69
1.86
304 (4.50), 356 (4.33), 495 (4.38) 583 (-)
304 (4.68), 460 (4.24), 556 (4.20) 634 (79)
352 (4.55), 464 (4.10),586 (4.17) 673 (30)
311, 466, 564, 607 (645)
312, 595 (695)
360 (4.68), 628 (4.25)
370 (4.61), 648 (4.11)
721 (6)
745 (1)
366, 630, 688 (752)
372, 675, 745 (798)
355, 555, 646 (701)
361 (4.64), 556 (4.38), 640 (4.18) 682 (13)
a In chloroform (1 ꢀ 105 M). b In toluene estimated by using the comparative method with cresyl violet as standard fluorophore (Φ = 67% in
methanol). c Spincoated on a glass substrate from a 10 mg/mL toluene solution (thikness 120 nm). d 0.1 mol dm-3 of n-Bu4NPF6, in THF, Pt electrode,
3
scan rate 50 mV s-1
calculations (B3LYP/6-31G*).
.
e Calculated ELUMO = -(ERed,onset - E(1/2)
þ 4.8) eV. f Calculated from Eopt - ELUMO
.
g DFT quantum mechanical
Fcþ/Fc
series 7a, 8a, and 9a, the quantum yield17 decreased from
79% (7a) to 30% (8a) and to13% (9a). Theintroduction of
thiophene units in the molecule significantly reduced the
fluorescence quantum yield due to efficient intersystem
crossing induced by the heavy atom effect of sulfur.18
Additionally, in all three cases, the emission spectra dis-
played a strong solvent dependence (Figures S4-S6, SI).
Bathochromic shifts of 20 (7a), 31 (8a), and 23 nm (9a)
were registered from hexane to chloroform, evidencing a
more polar molecular structure in the excited state than in
the ground state. Hence, an efficient charge transfer from
the donor to the electron-deficient TQ core occurs. The
photophysical behavior in film was examined as well
(Figures S4-S6, SI). The broadening of the low-energy
absorption bands together with the red shift of the emis-
sion maxima propounded an effective π-π stacking in the
solid state. According to the CV investigations, all three
compounds exhibited 3-electron reduction reactions char-
acteristic for the TQ core, out of which the first was found
to be typically Nernstian and the other two were quasi-
reversible (Figure S7, SI, Table 1). The ELUMO levels, as
determined from the ERed, showed a slight decrease in the
series 7-9a. Furthermore, considering the direct correla-
tion between the LUMO levels and the Eop, the EHOMO
levels were calculated and listed in Tabel 1. The EHOMO
levels were most affected by the structural modification.
The increasing number of thiophene units (donor strength)
increased the EHOMO levels and thus lowered the band gap.
Finally, the photophysical and electrochemical impact
of the π-spacer (triple bond vs double bond) in the series
8a, 8b, and 8c, was evaluated (Table 1).
Figure 1. UV-vis absorption (solid line) and emission (dashed
line) spectra of the compounds 7-9a in chloroform.
All three compounds displayed strong absorption in
chloroform between 300 and 400 nm. In addition, two
bands were found around 450 and 556 nm for 7a and
around 450 and 586 nm for 8a, respectively. These two
additionalabsorption bands originatefromtheorthogonal
extension of the conjugation.15 In this series, compound
9a, bearing only thiophene substituents, showed the lowest
Eop. The long wavelength absorption was extended up to
690 nm with a maximum at 556 nm (shoulder at 640 nm).
Moreover, a weak solvatochromism (Δλmax) between hex-
ane and chloroform solutions was observed in this case
(Δλmax = 13 nm) (Figure S6, SI). Interestingly, as the
solvent polarity goes beyond chloroform, 9a exhibited a
negative solvatochromism (Δλmax from chloroform to
acetone was -15 nm). Such a phenomenon was previously
reported16 and was ascribed to the back electron transfer
from the acceptor side to the donor side in polar solvents.
All three compounds revealed red luminescence. In the
The absorption spectra in chloroform revealed two main
absorption bands for 8b and 8c (Figure 2). The first intense
band appeared around 365 nm with a broad shoulder at
435 nm and a second appeared at 628 (8b) and 648 nm (8c),
respectively. The systematic replacement of the ethynyl
units by a vinyl π-spacer gave rise to intense red shifts
of the lower energy absorption and fluorescence maxima.
Furthermore, the better structural organization in film is
(15) Boixel, J.; Blart, E.; Pellegrin, Y.; Odobel, F.; Perin, N.; Chiorboli,
C.; Fracasso, S.; Ravaglia, M.; Scandola, F. Chem.;Eur. J. 2010, 16,
9140.
(17) (a) Isak, S. J.; Eyring, E., M. J. Phys. Chem. 1992, 96, 1738. (b)
Williams, A.; Winfield, S. A.; Miller, J. N. Analyst 1983, 108, 1067.
(18) de Melo, J. S.; Burrows, H. D.; Svensson, M.; Andersson, M. R.;
Monkman, A. P. J. Chem. Phys. 2003, 118, 1550.
(16) Davies, J. A.; Elangovan, A.; Sullivan, P. A.; Olbricht, B. C.;
Bale, D. H.; Ewy, T. R.; Isborn, C. M.; Eichinger, B. E.; Robinson,
B. H.; Reid, P. J.; Li, X.; Dalton, L. R. J. Am. Chem. Soc. 2008, 130,
10565.
1938
Org. Lett., Vol. 13, No. 8, 2011

ted the experimently determined observations according to
which, employing ethynyl spacers slightly decreased the
LUMO energy levels of the compounds (see 7a, 8a, and
9a), while vinyl spacers increased both HOMO and LUMO
levels. In total, however, it led to lower energy gaps (see 8a,
8b, and 8c) (Figures S10 and S11, SI).
In conclusion, by attaching a strong donor at posi-
tions 6 and 7 to the TQ core its high electron affinity was
not altered, but the energy gap was significantly reduced.
Furthermore, the photophysical and electrochemical prop-
erties of these new TQ based molecules were tunable by
the nature of the donor moiety and π-spacer at positions
4 and 9. By extending the conjugation through D-acetylene
spacer low-energy absorbing and red-emitting molecules
were accessible having low-lying LUMO energy levels.
Additionally, their good solubility in common organic
solvents and good thermal stability (Figure S12, SI)
render these systems particularly attractive for organic
photovoltaics or field effect transistors. Furthermore,
the ethynyl spacer could be easily reduced to a vinyl
spacer, in a one-pot fashion, by increased reaction time.
These changes led to an extended intramolecular con-
jugation for fine-tuning of the photophysical and elec-
trochemical properties.
Figure 2. UV-vis absorption (solid line) and emission (dashed
line) spectra of the compounds 8a-c in chloroform.
illustrated by the significant red shift of both absorption
and emission in film relative to solution (Figure S8, SI).
The ELUMO levels were determined from the reduction
potentials and listed in Table 1 (Figure S9, SI). The
increase of the ELUMO could be assigned to the stronger
electron-donating property of the vinyl spacer compared
to the ethynyl spacer. Moreover, the EHOMO levels derived
from the optical band gap were increased with the intro-
duction of vinyl spacers.
Quantum mechanical calculations by density functional
theory (DFT), using B3LYP functional with 6-31G* basis
set, were employed toestablishthe geometry and electronic
structure of the presented molecules (Figure S10, SI). The
calculations revealed that in all compounds the LUMO
orbitals are localized on the TQ core and the HOMO levels
reside along the conjugated backbone. Although the cal-
culated energy levels were higher than the ones determined
experimentally, the same trend was ascertained. The pre-
dicted energy gaps were in good agreement with the optical
ones (Table 1). The results of the DFT calculations suppor-
Acknowledgment. Financial support by the European
Union within the SolarNtype (MRTN-CT-2006-03553)
project is gratefully acknowledged. We also thank Dr.
Simon Stelzig (Max Plank Instutute for Polymer Research,
Mainz) and Dr. Giorgio Zoppellaro (Department of Mo-
lecular Biosciences, University of Oslo) for fruitful discus-
sions. We are indebted to Dr. Norbert Hanold (Institute of
Organic Chemistry, University of Mainz) for carrying out
the ESI measurements.
Supporting Information Available. Synthetic proce-
dures, NMR, MALDI-TOF, ESI, IR, photophysical, and
electrochemical data, computational details, and MW
reaction profiles. This material is available free of charge
via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 8, 2011
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